Don't know about the weight. But look at what other sellers are asking for postage costs on similar saddles and you should get an idea what you should ask too. Don't go too high or it may put people off.
When I've sold saddles it's been same price for up to 10kg, which most saddles are under. I send them on next day recorded and it's about £21 so I charge £25 for P & P as I use loads of bubble wrap.
ETA I never book them in, just take box to PO and they weigh it there.